Description of Related Art
The following is a general description of art relevant to the present invention. None is admitted to be prior art to the invention.
All organisms possess the ability to detect and degrade RNA transcripts that contain a premature translation stop codon. Such RNA transcripts are termed "nonsense" transcripts, and the cellular process of degrading these transcripts is termed nonsense-mediated RNA decay (NMRD). Although has been studied extensively in yeast mammalian factors that participate in NMRD have not been described previously. In yeast, the protein Upf1, encoded by the Upf1 gene, is thought to be involved in NMRD(Leeds et al., 1992, Csaplinski et al., 1995). Upf1 shares a high degree of sequence homology with Sen1 of S. cerevisiae, which mediates tRNA splicing, and Mov-10, which is a mouse protein of unknown function. Two additional factors in yeast, Upf2 and Upf3, also are essential for NMRD.
Several factors suggest that NMRD differs substantially between yeast and mammals. For example, while translation is thought to be involved in yeast, several observations suggest that translation does not play a similar role in mammalian cells (Peltz et al., 1994). First, for many genes and mutations, the reduction in the level of nonsense transcripts in the cytoplasm is coupled with a similar reduction in the level of nonsense transcripts in the nucleus of the cell. Examples of transcripts that are reduced in level in both the cytoplasm and nucleus, despite normal rates of transcription, include transcripts encoding .beta.-globin, DHFR, TPI, MUP, and avian sarcoma virus src. A second indication that mammalian NMRD may differ from yeast NMRD is the suggestion that NMRD occurs in the nucleus, rather than cytoplasm, of mammalian cells (Maquat, 1995). The factors that suggest that NMRD occurs in the nucleus of mammalian cells include the observations that: (i) NMRD is dependent upon intron sequences; (ii) most transcripts derived from intronless minigenes are immune to NMRD; (iii) once nonsense transcripts reach the cytoplasm and are bound by polysomes, they display a normal degree of stability; and (iv) nonsense codons influence pre-mRNA processing. A particularly compelling observation is that nonsense mutations can alter the ability of a cell's RNA splicing machinery to recognize exons; this process occurs within the nucleus, and is not dependent upon ribosomes (Dietz et al., 1993a; Dietz and Kendzior, 1994).
NMRD has a distinct physiologic role in intracellular processing, that is, to reduce the presence of abnormal proteins within cells. In the absence of NMRD, premature termination codons (PTCs) that result from nonsense mutations can lead to the production of proteins which are truncated versions of wild-type proteins. PTCs resulting from frameshifts, gene rearrangements, or the retention of introns can, in the absence of normal functioning NMRD, produce aberrant proteins which differ in sequence from the wild-type protein. Such deviant proteins are implicated in disease states. It is anticipated that mammalian organisms containing mutations in RENT1 are likely to suffer from syndromes associated with the excessive build up of these abnormal proteins such as Marfan Syndrome, accelerated aging or various cancers. Thus, RENT1 plays an important role in the control of NMRD and consequently for the prevention of disease states due to the expression of mutant proteins.